1. Field of the Invention
The present invention concerns a method to generate a magnetic resonance angiography image of a vascular structure of an examination region, and MR system for implementing such a method.
2. Description of the Prior Art
For the depiction of vessels with magnetic resonance tomography (MRT), methods are used in which contrast agent is injected into an examined person, the propagation of which contrast agent in the body then being detected. Furthermore, angiography techniques implemented without contrast agent are known in which the influence of flowing spins on the MR signal is used. In this angiography technique operating without contrast agent, a technique known as spin labeling (i.e. a marking of the spins) is used, among other things. A slice-shaped volume that includes the vessels to be depicted is hereby marked with the use of a slice-selective saturation pulse. This volume forms the saturation volume that is excited by radiation of one or more RF saturation pulses.
The term “saturation pulse” is generically used in the following as a designation for pulses that are used to magnetically mark the saturation volume. They can be (but do not necessarily need to be) pulses with an excitation angle of 90 degrees which, in combination with what are known as spoiler gradients, lead directly to a saturation of the volume. The term “saturation pulse” also encompasses other types of pulses that (for example) lead to a saturation of the magnetization at later points in time, such as inversion pulses with a target flip angle of 180 degrees, for example.
After excitation of the spins in the saturation volumes, the spins are detected after an inversion time period TI, with the inversion time period being selected such that the inverted spins ideally contribute no signals to the total signal in the signal acquisition. In comparison to the saturated spins, the spins flowing from outside the saturation volume into the volume via the feed (supply or delivery) vessels have a high signal strength in the signal acquisition, which is then used to generate the MR angiography image.
However, in the case of examined persons with a low cardiac output, or in regions with slow blood flow, it is difficult to sufficiently fill the vessel tree of interest with fresh, unsaturated, inflowing blood, particularly if the inversion time period is short. Those portions of the vessel tree that are not reached by the fresh, unsaturated spins remain dark in the arising MR angiography image. Due to the preceding inversion, the blood that flows from the saturation volume into the vessel tree generates nearly no signal, and therefore shortens the length of the portion of the vessel tree that is visible within the MR angiography images.
In order to ensure the visibility of optimally the entire vessel tree, up to the peripheral branchings of the vessel structure, even for patients with a low cardiac output, it is sought to place the interface between the saturated volume and the unsaturated volume as close as possible to the vessel tree to be examined.